1. Field of the Invention
The invention relates to a process for the manufacture of a sheet or film made of an impact-resistant polymer such as polycarbonate or polymethyl methacrylate with a scratch-resistant, soft highly flexible surface coating made of a completely transparent polyurethane by applying reaction mixtures containing an isocyanate component and a polyol component, on the sheet or film.
2. Discussion of the Background
It is known to provide sheets or films made of a scratch-sensitive polymer with a soft scratch-resistant layer made of a polyurethane having self-healing properties. This soft scratch-resistant layer can be manufactured either as a prefabricated film and be laminated on the sheet or film by inserting an adhesive layer. It can also be produced on the surface of the sheets or films through direct application of a reaction mixture reacting completely into the polyurethane layer (DE-OS 20 58 504).
If these soft polyurethane layers are to fulfill their function as a scratch-resistant layer, specific demands with respect to their scratch resistance and their functional properties are made of these layers. Polyurethane layers which have these properties are known. However, it has been demonstrated that these known polyurethane compositions are not suitable for the coating of polymers and in particular, for the coating of sheets or films made of polycarbonate, which are to be used, for example, as interior cover layers on glass for motorized ground, air and naval vehicles or building window glass panes. In particular, the known scratch-resistant layers of this kind have the drawback that they adhere poorly to the polycarbonate. It was been proposed to increase the adhesion of the soft polyurethane layers on the polycarbonate through the addition of inorganic chromium salts, but it has been shown that the improved adhesion attempted in this manner is inadequate. In addition, chromium salts are toxic and only slightly soluble in the reaction mixture.
Especially high demands are made of the adhesion between the soft scratch-resistant layer and the polycarbonate film in those cases in which a coated polycarbonate sheet is attached to a glass pane in order to manufacture a breakthrough or bulletproof laminated glass pane at raised temperatures and raised pressures. Here, the initial layer arrangement, at a pressure of about 10 atm., is subjected to a temperature of about 140.degree. C. The bonding process produces significant mechanical stresses within the layers. It has been demonstrated that the adhesion between the polycarbonate sheet and the soft, scratch-resistant coating according to prior art is weakened during this heat-pressure treatment so that it cannot withstand subsequent tests. If scratch-resistant coatings based on solvent-containing systems are used, there is the added feature that with just the smallest amount of residual solvent the polycarbonate shows marked stress cracking.